As computer hardware and software technology continues to progress, the need for larger and faster mass storage devices for storing computer software and data continues to increase. Electronic databases and computer applications such as multimedia applications require large amounts of disk storage space.
To meet these ever increasing demands, the hard disk drive (HDD) continues to evolve and advance. Some of the early disk drives had a maximum storage capacity of five megabytes and used fourteen inch platters, whereas today's HDDs are commonly over one gigabyte and use 3.5 inch platters. Advances in the amount of data stored per unit of area, or areal density, have dramatically accelerated. For example, in the 1980's, areal density increased about thirty percent per year while in the 1990's annual areal density increases have been around sixty percent. Areal density may be increased by increasing the rate at which data may be stored and retrieved. The cost per megabyte of an HDD is inversely related to its areal density.
In general, mass storage devices and systems, such as HDDs, include a magnetic storage media, such as rotating disks or platters, a spindle motor, read/write heads, a servo actuator, servo circuitry, a pre-amplifier, a read channel, a write channel, a memory, and control circuitry to control the operation of the HDD and to properly interface the HDD to a host or system bus. The read channel, write channel, servo circuitry, and memory may all be implemented as one integrated circuit that is referred to as a data channel. The control circuitry often includes a microprocessor for executing control programs and processing information provided during the operation of the HDD.
An HDD performs write, read, and servo operations when storing and retrieving data. A typical HDD performs a write operation by transferring data from a host interface to its control circuitry. The control circuitry then stores the data in a local dynamic random access memory (DRAM). A control circuitry processor schedules a series of events to allow the information to be transferred to the disk platters through a write channel. The read/write heads are moved to the appropriate track and sector. Finally, the HDD control circuitry transfers the data from the DRAM to the sector using the write channel. A sector generally has a fixed data storage capacity, such as 512 bytes of user data per sector. A write clock controls the timing of a write operation in the write channel. The write channel may encode the data so that the data can be more reliably retrieved later.
In a read operation, the appropriate sector to be read is located and data that has been previously written to the disk is read. A read/write head senses the changes in the magnetic flux of the disk platter and generates a corresponding analog read signal. The read channel receives the analog read signal, conditions the signal, and detects "zeros" and "ones" from the signal. The read channel conditions the signal by amplifying the read signal to an appropriate level using an automatic gain control circuit. The read channel then filters the signal, to eliminate unwanted high frequency noise, equalizes the channel, detects "zeros" and "ones" from the signal, and formats the binary data for the control circuitry. The binary or digital data is then transferred from the read channel to the control circuitry and is stored in the DRAM. The processor then communicates to the host that data is ready to be transferred. A read clock controls the timing of a read operation in the read channel. The goal during a read operation is to accurately retrieve the data while minimizing the bit error date (BER) in the noisiest environment.
Recently, advanced techniques utilizing discrete time signal processing (DTSP) to reconstruct and read the original data written to the disk are being used in read channel electronics to improve areal density. In these techniques, the data is synchronously sampled using a data recovery clock. The sample is then processed through a series of mathematical manipulations using signal processing theory. There are several types of synchronously sampled read channels. Partial response, maximum likelihood (PRML); extended PRML (EPRML); enhanced, extended PRML (EEPRML); fixed delay tree search (FDTS); and decision feedback equalization (DFE) are several examples of different types of synchronously sampled read channels using DTSP techniques. The maximum likelihood detection performed in several of these systems is usually performed by a Viterbi decoder implementing the Viterbi algorithm, named after Andrew Viterbi who developed it in 1967.
In a servo operation, the servo circuitry generates a track identification signal and a position error signal (PES) by reading and demodulating a servo wedge stored on each sector. The PES indicates the relative alignment of the read/write head on a particular track so that the head may be properly positioned for both read and write operations. The servo wedge includes track identification information, for generating the track identification signal, and track misregistration or position error information, for generating the PES. The position error information may be provided as servo bursts. The track identification signal and PES are provided to the control circuitry during read and write operations so that a track may be identified by the track identification signal and the read/write heads may be properly aligned on the track using the PES.
Traditionally, servo circuitry includes peak detection circuitry that is used during a servo operation to assist with processing the servo wedge so that the track identification signal and PES may be generated. Problems arise when using peak detection circuitry that ultimately result in reduced overall HDD capacity, excess power consumption, and increased track rereads that harm overall HDD performance. The overall HDD capacity is reduced because the peak detection circuitry is relatively slow in processing the servo wedge and generating the corresponding track identification signal and PES. Because of this, a larger servo wedge, requiring additional HDD capacity, must be provided to the peak detection circuitry. This reduces the overall HDD capacity available for data/information storage. Power consumption is increased because of the additional circuitry needed to implement the peak detection circuitry. Power consumption is especially critical in portable or battery powered applications such as laptop or notebook computers. The additional circuitry may also increase overall fabrication costs. Peak detection circuitry may not be as accurate as desired resulting in track identification signal errors. Track identification signal errors result in track rereads which harm overall HDD performance.